In 45 BCE Julius Caesar, upon the advice of the Greek astronomer Sosigenes, decided to use a purely solar calendar. This calendar, known as the Julian calendar, fixed the normal year at 365 days, and put the leap year every fourth year, at 366 days. Leap year is so named because the extra day causes any date after February in a leap year to "leap" over one day in the week and to occur two days later in the week than it did in the previous year, rather than just one day later as in a normal year.

The Julian calendar also established the order of the months and the days of the week as they exist in the Gregorian calendar. In 44 BCE Julius Caesar changed the name of the month Quintilis to Julius (July), after himself. The month Sextilis was renamed Augustus (August) in honor of the Roman emperor Caesar Augustus, who succeeded Julius.

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